A photomask may be used to transfer a pattern to a semiconductor wafer. The pattern, which is transferred onto the wafer, may be first formed on a photomask substrate that is substantially transparent. The substrate includes thin films of metal or other nontransparent material, which act to block light passing through the photomask substrate. In this way, a pattern is transferred onto the semiconductor wafer for use during semiconductor processing. In particular photomasks, the nontransparent material may be a chrome material, such as opaque chrome.
Due to limitations imposed by the wavelength of light used to transfer the pattern, resolution at the edges of the patterns of the photomask degrades when ordinary optical techniques are used. Standard optical techniques using ultra violet (UV) light may extend the lower range, but still fall short of desired resolution at low ranges (under 0.25 microns). Use of phase-shifting photomasks is one method of increasing the resolution of patterns by creating phase-shifting regions in the transparent areas of a photomask.
Standard phase shift masks generally are formed either by depositing transparent films of appropriate thickness and patterning them over the desired transparent areas using a second level lithography and etch technique, or by etching vertical trenches in the substrate. In both of these instances, the "edges" or "walls" between the phase shifted and unshifted regions generally result in a transition between high and low refractive index regions. This transition of the refractive index and the three dimensional structure causes scattering of light due to internal reflections at the edges and causes the transmitted light intensity and spatial profile to vary between the shifted region and the unshifted region.
One method of forming a phase-shifting photomask is to utilize a material that is substantially nontransparent, such as opaque chrome to define the device pattern. This pattern may be fabricated using standard lithography processes. For example, a layer of chrome may be deposited on a quartz substrate by known techniques. Then a resist layer may be formed on the substrate and patterned using standard lithography processes. The patterned resist creates openings which expose the chrome layer. Then an etching process is performed in which openings are created that expose the quartz layer. Then the resist layer is removed and a second layer of resist is applied. This layer may be patterned to expose certain portions of the quartz, while covering other portions. Next, an etching step is performed which creates trenches in the exposed portions of the quartz layer. Finally, the remaining resist may be stripped from the substrate. The trenches formed in the quartz thereby effectively shift light 180.degree., and thereby increase the resolution of patterns created on semiconductor wafers by the photomask. In certain embodiments, it may be possible to deposit an oxide layer over the patterned chrome layer, and then pattern this oxide layer.
Several problems exist with this method of fabricating phase-shifting masks. First, the resist layers may contain pinholes. These pinholes may be any size. Presence of pinholes can create defects within a photomask. For example, a resist layer is used to block a lower layer during an etch process. The etch process will impinge and cause the etchant chemicals to flow into the pinhole, thereby etching an undesired portion of a lower layer. Another problem is that unwanted resist residues may remain on the photomask. A method of reducing pinholes is to improve the resist quality; however this does not completely prevent the presence of pinholes.